Method for producing a melt carbonate-fuel cell and to melt carbonate fuel cells

ABSTRACT

The invention relates to a method for producing a melt carbonate fuel cell comprising a cathode layer made from porous nickel oxide, an anode layer made from porous nickel and a melt arranged between the cathode layer and the anode layer, received in the form of a finely porous electrolyte matrix melt consisting of one or more alkali metal carbonates as electrolytes. In order to produce the cathode layer, a sintered, coated electrode path, coated with catalytically activating particles, made of porous nickel in the fuel cell operation mode is reacted to form nickel oxide. According to the invention, the electrode path is coated with catalytic activating particles made from one or more non-oxidic inorganic metal compounds, which are reacted to form the corresponding metal oxides under gas development. The invention relates to another similar fuel cell with increased activation of the cathode reaction.

[0001] The invention relates to a method for producing a moltencarbonate fuel cell as well as to a molten carbonate fuel cell.

[0002] Fuel cells are devices in which a chemical reaction takes placebetween a gas and an electrolyte. In principle, in the reverse of theelectrolysis of water, a hydrogen-containing fuel is brought up to ananode and an oxygen-containing cathode gas is brought up to a cathodeand converted to water. The energy released is removed as electricalenergy.

[0003] Molten carbonate fuel cells (MCFC) are described, for example, inDE 43 03 136 C1 and DE 195 15 457 C1. In their electrochemically activeregion, they consist of an anode, an electrolyte matrix and a cathode.As electrolyte, a melt of one or more alkali metal carbonates is used,which is placed in a finely porous electrolyte matrix. The electrolyteseparates the anode from the cathode and seals the gas spaces of theanode and cathode from one another. During the operation of a moltencarbonate fuel cell, a gas mixture, containing oxygen and carbondioxide, generally air and carbon dioxide, is supplied to the cathode.The oxygen is reduced and the carbon dioxide is converted to carbonateions, which migrate into the electrolyte. Hydrogen-containing fuel gasis supplied to the anode, the hydrogen being oxidized and converted withthe carbonate ions from the melt into water and carbon dioxide. Thecarbon dioxide is recycled to the cathode. The oxidation of the fuel andthe reduction of the oxygen take place separately from one another. Theoperating temperature is between 550° and 750° C. MCFC cells transformthe chemical energy, stored in the fuel, directly and efficiently intoelectrical energy.

[0004] A generic method for producing such a molten carbonate fuel cellis described in the DE 43 03136 C1. Usually, a slurry of nickel powderof a particular particle size and various auxiliary materials isprepared, pulled out into an electrode web or sheet and dried. Theelectrode web is formed into serviceable electrode material in that itis heated, freed from organic components and sintered. The resulting,sintered, porous nickel web is incorporated in fuel cells. The fuel cellis heated to its operating temperature, a cathode layer of nickel oxidebeing formed by the action of the molten electrolyte. Since theelectrolyte generally contains lithium, the nickel oxide layer is dopedwith lithium oxide (lithiated). During the operation of the cell, thereis a thin electrolyte film on the surface of the cathode material, inwhich the transport and the chemical reactions of the electrochemicallyactive species take place. The performance of the cathode is affected toan appreciable extent by its morphology, the necessary gas permeabilitybeing ensured by a correspondingly high porosity of, for example, morethan 60 percent during the operation of the cell.

[0005] However, the gas permeability through the cathode and theelectrochemical reactions at the cathode surface is inadequate forhigher cell outputs, so that the electrode must be activatedcatalytically. One possibility for activating the cathode consists ofcoating the surface of the cathode with transition metal oxides such ascerium oxide, titanium oxide or zirconium oxide, the particles generallybeing finely dispersed at the surface. The particle size of theactivating species must be small enough to achieve an adequately highsurface area.

[0006] It is a problem that the cathodes, in the unoxidized state, thatis, when they consist essentially of nickel, must be coated with thecatalytically activating particles, since the oxidation of the nickel tonickel oxide takes place during the operation of the fuel cell, that is,after the incorporation of the components in the fuel cell. However,since the oxidation of nickel to nickel oxide is associated with adrastic increase in volume, the bulk of the particles are overgrown bynickel oxide, so that these particles no longer are available for thecatalytic activation.

[0007] U.S. Pat. No. 4,430,391 is concerned with cathodes for fuelcells. Their catalytic activity is increased by a selective change inthe microstructure of the cathode material, so that locally disorderedregions, which are not in equilibrium, are formed. This is, however,very cumbersome.

[0008] According to DE 42 35 514 C2, the nickel of the cathode of amolten carbonate fuel cell is protected by an electrochemically activeof a double oxide before leaching by the molten carbonate electrolytes.The coating contains, for example, nickel, iron, cobalt or titanium. Toproduce the coating, a porous, pre-sintered nickel oxide matrix, withthe help of a conventional coating method, is provided with anessentially non-oxide layer of the double oxide, which is to be formed,and then converted by heating under oxygen or by the cell operation intothe double oxide form. It is furthermore described that the electrode isincorporated into the cell with a metal layer or metal hydroxide layer.In one example, a cathode is used, which is not oxidized and isimpregnated with cobalt nitrate.

[0009] DE 689 01 782 T2 discloses that the electrode for a moltencarbonate fuel cell may be impregnated with a compound, which isconverted by a heat treatment into a ceramic material. The startingmaterial is an oxide, carbide, nitride, boride or nitrate, whichcontains, for example, aluminum or zirconium. The heat treatment oroxidation evidently takes place before the oxidation of the nickelmaterial of the cathode.

[0010] It is an object of the present invention to provide a method forthe preparation of a fuel cell, as well as a fuel cell of theabove-mentioned type, which has a better catalytic activity.

[0011] The objective is accomplished by a method with the distinguishingfeatures of claim 1 and by a fuel cell with the distinguishing featuresof claim 8. Pursuant to the invention, the electrode web is coated withcatalytically activating particles of one or more inorganic metalcompounds, which are not oxides and which are converted during theoperation of the fuel cell to the corresponding metal oxides withgeneration of gas.

[0012] The use of such particles of inorganic metal compounds, which,like the cathode, are converted to the corresponding oxides and releasegas only during the operation of the fuel cell, has the advantage thatthe resulting metal oxides on the cathode surface cannot be overgrown bythe nickel oxide formed and subsequently are exposed. The gas, escapingin small bubbles, forces the nickel oxide, which is formed, back aroundthe metal oxide particle, so that, in addition, fine pores are formed,in which the metal oxide particle is embedded and has at least one freesurface. Accordingly, all or at least the bulk of the metal oxideparticles is available for activating the cathode reaction.

[0013] Further advantages of the invention are described in thedependent claims.

[0014] Preferably, inorganic metal compounds are used which, during thereaction to the corresponding metal oxides, release nitrogen and/orcarbon dioxide as gas. They include, in particular, metal carbides,metal nitrides and metal carbonitrides.

[0015] Suitable metals are, for example, titanium, zirconium, cerium,iron, cobalt, aluminum and nickel, the use of titanium, zirconium andcerium being preferred. Preferably, titanium nitride, titanium carbide,titanium carbonitride, zirconium nitride, zirconium carbide, ceriumcarbide and cerium nitride are used, all or which are commerciallyobtainable. In principle, the activation can also be attained with othermetal carbides, nitrides or carbonitrides, since the generation of gasand the formation of pores can also be obtained with them. It is onlyimportant that the resulting metal oxides are stable when in contactwith the alkali metal carbonate melt and cannot contribute to thepoisoning of the electrolyte.

[0016] Preferably, between 0.001% by weight and 0.5% by weight ofnon-oxide metal carbides and/or non-oxides metal nitrides and/ornon-oxides metal carbonitrides, based on the weight of the electrodeweb, are used. Small particles are used in order to achieve the largestpossible metal oxide surface and, with that, a satisfactory activationof the cathode reaction.

[0017] The cathodes can be produced by conventional manufacturingprocesses (such as dry doctoring or tape casting), which are known tothose skilled in the art and are also described in the documents namedabove, which outline the state of the art. Since the non-oxide,inorganic metal compounds, at least the carbides and nitrides, arestable in the sintering atmosphere and are decomposed only at highoxygen partial pressures when the fuel cell is in operation, it ispossible to coat the electrode web before the sintering with thecatalytically activating particles.

[0018] In the following, the invention is explained in even greaterdetail by means of the attached drawings, in which

[0019]FIG. 1 diagrammatically shows the construction of the activecomponents of a fuel cell,

[0020]FIG. 2 diagrammatically shows the position of the metal particlesin a nickel oxide cathode, which is produced by a conventional methodand

[0021]FIG. 3 diagrammatically shows the position of the metal oxideparticles of a nickel oxide cathode, which is produced according to theinventive method.

[0022] In FIG. 1, the electrochemically active components of a fuel cellare shown diagrammatically, namely the anode 1, the electrolyte matrix 2and the cathode 3. The electrolyte matrix may, for example, be an LiAlO₂matrix, filled with lithium-containing carbonates.

[0023] The cathode is produced by conventional methods, such as theso-called “tape casting” or “dry doctoring” method. In general, a slurryof nickel powder of a particular particle size and various auxiliarymaterials is produced, drawn out to an electrode web or sheet and dried.The electrode web is formed into a serviceable electrode material, inthat it is heated, freed from organic components and sintered. Theresulting, sintered porous nickel web is incorporated in fuel cells. Thefuel cell is heated to its operating temperature, a cathode layer ofnickel oxide being formed due to the action of the molten electrolyte.

[0024] The particles of activating materials are applied on theelectrode web before the incorporation in the fuel cell. In conventionalmethods, these particles are metal oxides. In FIG. 2, it is showndiagrammatically what subsequently happens during the conversion of thenickel to nickel oxide. A nickel particle 10, which carries a metaloxide particle 12 at its surface 11, is shown at the left. After theconversion to nickel oxide in situ, a nickel oxide grain 20 is obtained,which is clearly larger than the nickel grain 10. The bulk of the metaloxide particles 12 is enclosed all around by nickel oxide (FIG. 2 at thebottom right), so that they are no longer available for activation ofthe cathode reaction. Only a small portion of the metal oxide particles12 remains at the surface 21 of the nickel oxide grain (FIG. 2, topright).

[0025] Pursuant to the invention, the electrode web is coated withnon-oxide, inorganic metal compounds only before or after the sintering.During the reaction of nickel to nickel oxide in situ, metal carbides,for example (such as titanium carbide (TiC)), is converted to metaloxides (in the case of titanium carbide, to titanium dioxide) in thefollowing manner:

MC+2O₂→MO₂+CO₂  (I)

[0026] for example,

TiC+2O₂→TiO₂+CO₂

[0027] Metal nitrides (such as titanium nitride) are converted, asfollows, to metal oxides (in the case of titanium nitride to titaniumdioxide):

MN+O₂→MO₂+0.5N₂  (II)

[0028] for example,

TiN+O₂→TiO₂+0.5N₂

[0029] The carbon dioxide or nitrogen released tears open the nickeloxide and locally forms new surfaces, the metal oxide being exposed.This is shown diagrammatically in FIG. 3. At the left, a nickel grain 10is shown once again. At its surface 11, it carries a particle 13 of anon-oxide, inorganic metal compound. At the right, it is seen that,after the reaction to nickel oxide in situ, the resulting nickel oxidegrain 20 has, aside from its regular surface 21, new additional surfaces22, 23. The metal oxide particle 12, which is also formed in situ, ispartially exposed.

What is claimed is:
 1. A method for the preparation of a moltencarbonate fuel cell with a cathode of porous nickel oxide, which iscoated with catalytically activating particles, an anode of porousnickel and a melt of one or more alkali metal carbonates as electrolyte,which is disposed between the cathode and the anode and taken up in afinely porous electrolyte matrix, the cathode being produced from asintered electrode web of porous nickel, which is converted in situ tonickel oxide during the operation of the fuel cell, wherein theelectrode web, before it is incorporated in the fuel cell, is coatedwith particles of one or more non-oxide inorganic metal compounds, andthe non-oxide inorganic metal compounds are reacted in situ during theoperation of the fuel cell with generation of gas to the correspondingcatalytically activating metal oxides essentially simultaneously withthe formation of the nickel oxide of the cathode web.
 2. The method ofclaim 1, wherein inorganic metal compounds are used which, during thereaction to the corresponding metal oxides, release nitrogen and/orcarbon dioxide as a gas.
 3. The method of claim 2, wherein metalcarbides and/or metal nitrides and/or metal carbonitrides are used asnon-oxide inorganic metal compounds.
 4. The method of one of thepreceding claims, wherein the metal or metals are selected from thegroup comprising titanium, zirconium, cerium, iron, cobalt, aluminum andnickel.
 5. The method of claim 4, wherein the particles are selectedfrom the group comprising titanium nitride, titanium carbide, titaniumcarbonitride, zirconium nitride, zirconium carbide, cerium carbide andcerium nitride.
 6. The method of one of the preceding claims, whereinbetween 0.001% by weight and 0.5% by weight of non-oxide metal carbidesand/or non-oxide metal nitrides and/or non-oxide metal carbonitrides areused, based on the weight of the electrode web.
 7. The method of one ofthe preceding claims, wherein the electrode web, before it is sintered,is coated with the catalytically activating particles and the sinteringpreferably takes place in a reducing atmosphere.
 8. A fuel cell with acathode of porous nickel oxide, an anode of porous nickel and a melt ofone or more alkali metal carbonates as electrolyte, which is disposedbetween the cathode and the anode and taken up in a finely porouselectrolyte matrix, the cathode layer, on its surface facing theelectrolyte matrix, being coated with catalytically activatingparticles, wherein essentially all catalytically activating particlesare exposed on the surface.
 9. The fuel cell of claim 8, wherein theparticles are embedded in additional pores at the surface of thecathode.
 10. The fuel cell of claims 8 to 9, wherein the particles areoxides of one or more metals from the group comprising titanium,zirconium, cerium, iron, cobalt, aluminum and nickel.